The homologous recombination (HR) pathway plays a critical role in the repair of the most harmful DNA lesions, DNA double-strand breaks (DSB) and inter-strand cross-links (ICL). Malfunction of HR causes genome instability, cancer, and genetic diseases. RAD51, a key HR protein, promotes DNA strand exchange, a basic step of HR. However, the mechanism of DNA strand exchange and specific functions of RAD51 in human cells remain to be elucidated. Traditional genetic approaches are difficult to apply because RAD51 is essential for cell viability. Here, we propose to develop small molecule inhibitors to study RAD51 activities and functions. These inhibitors may also be used as prototypes for development of combination therapies to sensitize cancer cells for radiation and chemotherapy. Because millions of cancer patients commonly undergo radiation therapy and chemotherapy, improvement of their efficacy will have a significant impact on the public healthcare. Small molecule compounds (MW < 500 Da) that can perturb specific functions of proteins became an important tool in modern biology. Small molecule inhibitors act rapidly and often reversibly. They can be introduced at any point of organism or cell development and applied in a dose-dependent manner, which is especially important in studies of proteins essential for cell viability, like RAD51. Previously, by high throughput screening we identified several small molecule inhibitors of RAD51 and demonstrated for the first time their biological activity in the cell. Here, we will use these inhibitors to analyze the mechanism of RAD51 DNA strand exchange and the functions of RAD51 in human cells. Using cellular and animal models we will explore the ability of the RAD51 inhibitors alone or in combination with inhibitors of othe DNA repair pathways to increase killing of cancer cells by chemotherapeutic agents.